The invention relates to an apparatus for rapidly measuring angle-dependent diffraction effects on finely patterned surfaces in the submicron range.
In semiconductor fabrication, line widths and profiles of patterned layers often have to be monitored during the fabrication process. Complying exactly with the specifications for the line width is of crucial importance for the functionality of a product. In addition, other structural parameters such as trench depth or sidewall inclination are also of major importance. Suitable measuring apparatuses are required for monitoring these fabrication parameters on lithography masks, semiconductor wafers, or other very finely patterned surfaces.
With the extremely small structural widths in the region of 0.25 xcexcm that are used nowadays in semiconductor fabrication, conventional optical line width measuring apparatuses can no longer be used on account of diffraction and interference effects. Therefore, electron microscopes are used for measuring the profile of fine structures ( less than 1 xcexcm). On account of the stringent vacuum requirements, even very complex electron microscopes developed specifically for line width measurements have a comparatively low throughput. Consequently, after a fabrication step, only a small portion of the product wafers can be checked with regard to compliance with the process specifications. Furthermore, the long measuring times increase the probability of further batches being fabricated defectively until a process fault is detected. These dead times can cause major financial losses particularly in the case of the latest fabrication technologies with wafer diameters of 300 mm and very complex process steps. Moreover, only a small number of individual structures can be monitored on each wafer using electron microscopes, and so the measurements are not representative. Therefore, under certain circumstances, fabrication faults or the causes thereof will be identified only very belatedly. In order to check further structural parameters, so-called monitor wafers are additionally included (e.g. in deposition processes) in order to be able to determine layer thicknesses produced on unpatterned wafers or in order to enable the electron microscope to effect so-called cross section recordings, for which the semiconductor wafers must be destroyed. Primarily in the case of future large wafer diameters of 300 mm or more, these monitor wafers cause high costs, firstly on account of the pure material value and secondly because they significantly reduce the throughput of product wafers. In order to manage with the fewest possible monitor wafers and nevertheless to improve the product monitoring, what are required in semiconductor fabrication are cost-effective measurement methods for nondestructive and contamination-free checking of structural parameters on the product wafers. In this case, the measuring speed should be high enough that, e.g. after a critical process step, each product wafer can be monitored without significantly increasing the process time. Scattered light measurement offers one solution approach. In general, during this method, the measurement region to be examined is illuminated and the surface properties of the measurement region are inferred from the features of the reflected light. If there are periodic structures on the substrate and if coherent light is used, then diffraction and interference effects occur given a corresponding choice of wavelength. Said effects prevent a measurement in conventional optical apparatuses, but they are explicitly detected and evaluated in the case of scattered light measurement or diffraction analysis since they are characteristic of the structural parameters. In research, the so-called 2xcex8 method has acquired a certain importance in recent years. In this case, the angle of incidence of the measuring beam is varied within the plane of incidence and the intensities of the orders of diffraction are measured as a function of the angle of incidence. With the aid of complex model calculations, it is possible to determine from this diffraction measurement various structural parameters such as line width, trench depth or edge inclination. However, the measurement configurations used hitherto for this purpose are not very flexible or are comparatively slow, structurally complex and expensive.
In previous realizations, the light source is moved by precise mechanical components or the specimen to be examined is itself rotated about the measurement point. This raises the costs of the apparatus and restricts the range of use of the method. Lens systems produce different angles of incidence, so that all that has to be moved is an optical element (e.g. mirror or prism), not the specimen. However, only limited angles of incidence can be realized even with complex lens configurations having a large aperture. Under certain circumstances, disturbing reflections occur at the interfaces.
Simultaneously producing a plurality of angles of incidence with the aid of a multiple beam splitter (e.g. reflection grating) in conjunction with an ellipsoidal mirror is proposed in German Published Patent Application DE 198 24 624. However, the angles of incidence are chosen when the measuring apparatus is constructed and so this choice is fixed and the number of angles of incidence that can be realized simultaneously is limited. Principally, however, the diffraction effects are simultaneously superimposed on the specimen structures for the different angles of incidence. Consequently, an angle-dependent measurement of diffraction intensities is not possible. By contrast, the apparatus in accordance with DE 198 24 624 enables the simultaneous measurement under different diffraction angles, which may be advantageous when comparing the measurement with a single simulated diffraction pattern.
A measurement method should be nondestructive, free of contamination, fast, simple, and robust. A promising approach for determining structural parameters is offered by so-called scattered light measurement: the angularly resolved intensity measurement of light that has been scattered at a substrate. Reflected or transmitted light is diffracted in the case of specimens having periodic structures. Regular semiconductor structures, e.g. in memory modules, can be imagined as a reflective amplitude or phase grating. If the grating vector is situated in the plane of incidence, then, for a given angle xcex8î of incidence, the following grating equation holds true for the n-th order diffraction maximum with the angle xcex8n of reflection             sin      ⁢              xe2x80x83            ⁢              θ        i              +          sin      ⁢              xe2x80x83            ⁢              θ        n              =      n    ⁢          λ      g      
In this case, xcex describes the wavelength of the light used and g denotes the grating period. Accordingly, in addition to the direct reflection already present, higher-order diffraction maxima can arise if the wavelength of the light used is less than half the grating period. If the size of the structures examined lies in the region of the wavelength, then the simple scalar Fraunhofer diffraction equations can no longer be employed. Instead, a simulation of the intensity distribution requires the solution of the associated Maxwell""s equations with the boundary conditions applicable to the respective grating. Efficient numerical methods, such as e.g. the so-called rigorous coupled wave analysis, have been developed for this purpose in previous years. The nonlinearities that occur allow generally valid statements only to a very limited extent, for which reason the concrete individual case must always be considered or numerically calculated for the assessment of diffraction effects on small structures. In this case, the intensities and also the phases of the orders of diffraction depend on the properties of the incident beam (angle, polarization, wavelength), on the examined grating structure (grating periods, line width, line height, layer structure, edge rounding, roughness) and on the material properties of the substrate (refractive index, absorption index).
A diffraction analysis according to the prior art can be broken down into a plurality of steps. First of all, the intensity of the scattered or diffracted light is measured. Afterward, the measured intensity distribution is compared with a simulated intensity distribution, the grating parameters being varied in a predetermined range. This simulation of the diffraction process is effected on the basis of a priori knowledge of the specimen. Those parameter values that lead to the best correspondence between measurement and simulation are accepted as practical approximations of the real grating parameters.
It is accordingly an object of the invention to provide an apparatus for rapidly measuring angle-dependent diffraction effects on finely patterned surfaces that overcomes the hereinafore-mentioned disadvantages of the heretofore-known devices of this general type and that utilizes a simplified measuring construction that reduces the apparatus costs, enables use as an integrated measuring apparatus, and considerably accelerates the measurement.
With the foregoing and other objects in view, there is provided, in accordance with the invention, an apparatus for measuring angle-dependent diffraction effects. The apparatus includes a coherent radiation source producing coherent radiation; a device for deflecting rays of the coherent radiation in different directions; a concentric mirror reflecting the rays arriving differently and sequentially one after the other from the device for deflecting the coherent radiation at a focal point; and a detector unit for measuring the intensity of the radiation diffracted by a specimen having a measuring point located at the focal point.
In accordance with a further feature of the invention, the concentric mirror includes mirror segments disposed to act as a single concentric mirror.
In accordance with a further feature of the invention, device for deflecting the coherent radiation includes an electrically regulated rotating mirror, a micro-optical module, or an electro-optical module.
In accordance with a further feature of the invention, the concentric mirror has an axis of symmetry; and the device for deflecting the coherent beam is situated on the axis of symmetry of the concentric mirror. Moreover, the concentric mirror can have a center point; and the measurement point on the specimen is disposed on the axis of symmetry of the concentric mirror symmetrically and equidistantly from the center point of the concentric mirror as the device for deflecting the coherent beam.
In accordance with a further feature of the invention, the concentric mirror has an axis of symmetry; the device for deflecting the coherent beam is situated on the axis of symmetry of the concentric mirror; and the rotating mirror has an axis of rotation coinciding with the axis of symmetry and is always impinged by the coherent beam at a point on the axis of symmetry.
In accordance with a further feature of the invention, the concentric mirror has a center point; and the measurement point on the specimen is disposed on the axis of symmetry of the concentric mirror symmetrically and equidistantly from the center point of the concentric mirror as the device for deflecting the coherent beam.
In accordance with a further feature of the invention, the detector unit includes a photodiode linear array, a plurality of individual diodes, a single large-area photodiode, photodiode arrays, or CCDs.
In accordance with a further feature of the invention, the apparatus includes a mirror reflecting the radiation diffracted at the specimen onto the detector. The mirror for directing the diffracted radiation indirectly onto the detector unit can resemble the concentric mirror. Together, the mirror and the concentric mirror can form a mirrored ring.
In accordance with a further feature of the invention, the electrically regulated rotating mirror is for further deflecting the diffracted radiation onto the detector element.
In accordance with a further feature of the invention, the apparatus including a radiation detector impinged upon by a beam when the deflection device is specifically positioned.
In accordance with a further feature of the invention, the apparatus includes a beam splitter splitting the beam to create partial beams, and a radiation detector being impinged upon by a partial beam from the beam splitter when the deflection device is specifically positioned.
In accordance with a further feature of the invention, the apparatus includes a position sensitive device detecting a change in the angle of incidence through tilting and surface undulations of the specimen; and a piezoelement correcting for the change in the angle of incidence through tilting and the surface undulations of the specimen when measured data is evaluated.
In accordance with a further feature of the invention, the apparatus includes a CCD detecting a change in the angle of incidence through tilting and surface undulations of the specimen; and a piezoelement correcting for the change in the angle of incidence through tilting and the surface undulations of the specimen when measured data is evaluated.
In accordance with a further feature of the invention, the apparatus includes an xy measurement table positioning the specimen to measure different measurement points on a specimen.
With the objects of the invention in view, there is also provided an assembly for measuring angle-dependent diffraction effects. The assembly includes an apparatus as described above. In addition, the assembly includes a positioner for positioning the apparatus relative to the specimen.
In accordance with a newly developed method of diffraction analysis, the required computation complexity can be decisively reduced if an exact quantitative evaluation is dispensed with and only a classification is performed instead. A quantity of specimens is measured and classified in classes by recording the diffraction pattern for each specimen and measuring the specimen by other methods. The classification of feature or quality classes is carried out primarily according to the results of the other methods. However, care should be taken to ensure that the diffraction patterns of specimens within a class are similar, i.e. the diffraction patterns of the specimens have features that can be identified as similar. A diffraction measurement can thus be effected before another e.g. destructive analysis, for instance electron microscopy, which then leads to the classification. The diffraction patterns are thus assigned to the feature or quality classes. The processes of classifying feature or quality classes and finding features in the diffraction patterns for assessment of similarity belong to the preparations for the actual diffraction measurement of unknown specimens.
An unknown specimen is then classified by measuring specimens and assigning the diffraction pattern to a class. Such specimens are classified in quality classes using the measurements by other methods. Specimens within a class have similar diffraction patterns. It is possible to assign an unknown specimen to a class using its diffraction pattern and conclude that it has similar physical properties to the remaining members of this class.
The advantages of this method are that a simulation from a priori data is no longer necessary, and that the measurement and evaluation can be effected nondestructively and very rapidly during a current fabrication process using a simple measuring configuration. This makes it possible to examine a large number of specimens in a short time.
Compared to German patent application No. DE 198 24 624, the present invention provides, for the first time, simultaneous production of a plurality of angles of incidence with the aid of a multiple beam splitter (e.g. reflection grating) in conjunction with an ellipsoidal mirror. However, the angles of incidence are chosen when the measuring apparatus is constructed, and so this choice is fixed and the number of angles of incidence that can be realized simultaneously is limited. Principally, however, the diffraction effects are superimposed simultaneously on the specimen structures for the different angles of incidence. Consequently, an angle-dependent measurement of diffraction intensities is not possible.
On account of the large number of influencing variables, the grating parameters can be unambiguously determined or classified only if there are a sufficient number of intensity measurement values available for the measurement point examined. For this purpose, it is possible to vary one or more parameters of the measuring beam (angle, polarization, wavelength) and to measure the intensities as a function of the variable quantities. Thus, the diffraction analysis still can be used for structural characterization even if, owing to small grating periods, only few if any orders of diffraction occur apart from the direct reflection (see grating equation).
A device for measuring angle-dependent diffraction effects includes a coherent radiation source, a device for deflecting the coherent radiation in different directions, a mirror configuration that directs the deflected radiation to a specimen location, and a detector unit for measuring the intensity of the radiation diffracted at a specimen. The radiation deflected in different directions is reflected by the mirror configuration in such a way that the coherent beam is deflected onto the specimen with different angles of incidence in a temporally successively sequential manner.
For this purpose, the angle of incidence of the measuring beam is altered continuously or in small steps. The intensities of the direct reflection (zero-order diffraction) and also of the higher orders of diffraction that may occur are measured. The evaluation of the intensity profiles as a function of the varied angle of incidence allows conclusions to be drawn regarding the form and material of the periodic structures examined.
In the present invention, for the first time, use is made of an electrically rotating and regulated mirror (so-called galvanometer scanner) in conjunction with a stationary, non-planar mirror surface in order to vary the angle of incidence of the measuring beam for a 2xcex8 diffraction analysis of a fixed measurement point. The measuring configuration makes it possible to move to large angular positions within milliseconds with an accuracy of a few xcexcrad. As a result, it is possible to carry out a complete measurement with variation of the angle of incidence within a few tenths of a second. Furthermore, producing the different angles of incidence requires only a robust movable component (galvanometer scanner) and so the susceptibility to disturbance is reduced. The costs of the components used and also the space required for the measurement setup are comparatively small. Because the different angles of incidence are produced sequentially, exactly one incident measuring beam exists at each point in time. Consequently, superimposition of orders of diffraction does not occur. Irrespective of the type of structure examined, the intensities of all the diffraction maxima that can be detected by the detector can be measured and assigned. The approximately constant intensity of the measuring beam independently of the angle of incidence (in contrast to DE 198 24 624 A1) increases the measurement accuracy of the system. The same applies to the exclusive use of mirror surfaces instead of lenses with optical imaging errors.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in apparatus for rapidly measuring angle-dependent diffraction effects on finely patterned surfaces, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.